Diffusion pump



May 1969 HIROAKI OKAMOTO ET AL 3,442,440

DIFFUSION PUMP Filed April 21, 1967 Sheet of 2 FIG. 4

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TIME(mmute) INVENTOR; AlW-LJQ BY '2 MLWd1/M United States Patent 3,442,440 DIFFUSION PUMP Hiroaki Okamoto, Tokyo, and Yoshio Murakami, Yokohama-shi, Japan, assignors to Tokyo Shibaura Electric Co., Ltd., Kawasaki-shi, Japan, a corporation of Japan Filed Apr. 21, 1967, Ser. No. 632,821 Claims priority, application Japan, Apr. 28, 1966, 41/26,648 Int. Cl. F04f 9/00 US. Cl. 230-101 3 Claims ABSTRACT OF THE DISCLOSURE A diffusion pump for obtaining extremely stable high vacuum comprising a boiler to hold shallowly pump fluid, casing having input conduit connected to a chamber to be evacuated and output conduit connected to an auxiliary pump, a jet nozzle assembly positioned in the casing and open at the lower end to the boiler, and annular heater positioned around the upper periphery of the boiler.

Background of the invention This invention relates to a diffusion pump and more particularly to a diffusion pump for obtaining extremely stable high-vacuum.

As small-sized electronic devices of high performance are increasingly manufactured in recent years by means of vacuum deposition, stable high-vacuum of the order of 10- to 10" mm. Hg which is free from pressure fluctuations is more widely demanded of pumping systems of vacuum evaporators or the like for the manufacture of said electronic devices. Diffusion pumps are widely used in obtaining high vacuum of the aforementioned magnitude, because of their ability to easily produce a high pumping speed and their economic advantages. The principle of a diffusion pump consists in boiling and evaporating thoroughly purified low vapor pressure organic fluid or mercury in the boiler section of said pump, vigorously ejecting said vapor through a narrow jet nozzle towards the high pressure side to compress gas molecules diffused from the vessel to be evacuated, and pumping out said gas molecules by means of an auxiliary pump connected with said diffusion pump, while said vapor is subsequently cooled and liquefied by a condenser installed nearby, thus returning it into the boiler to evaporate in a closed cycle.

Many kinds of diffusion pumps utilizing the aforementioned principle have been used, for example, those having a plate heater provided at the bottom of the boiler or those having a heater submerged in the pump fluid in the boiler, and in any of them heat is always supplied from the bottom or the interior of the boiler. These diffusion pumps however have a common defect that, when they are operated under conditions approximating the ultimate vacuum of 10" to 10* mm. Hg, readings on the vacuum gauge widely fluctuate, and pulsations in the pressure occur, indicating the instability of the vacuum obtained. The inventors have found that these pressure bursts are caused by eruptive boiling arising within the boiler of the diffusion pump. Close observations of the phenomenon of the eruptive boiling and the pressure fluctuations have disclosed that when the liquid surface is quiet in a state preceding eruptive boiling, the evaporation rate is low, whereas the amount of evaporation increases substantially at the time of eruptive boiling, and causes turbulence in the ejected vapor streams, resulting in sharp changes in the pressure in a vacuum vessel. Pressure fluctuations also appear to arise when decomposition products of the pump fluid generated by overheating prior to eruptive boiling Patented May 6, 1969 A device has been developed as a means to prevent such eruptive boiling, and it comprises a removable cartridge type heater with several fins fitted to a heating cylin-der around the heater so as to improve the convection of pump fluid. In this device, however, the fin metal surfaces heated to elevated temperatures are exposed above the pump fluid surface, and are liable to cause the thermal decomposition of the pump fluid, thus causing pressure fluctuations. Therefore, such pump does not fully meet the purpose of obtaining a stable high vacuum, and moreover, this heating system is of complicated construction, being difiicultto apply to fractionating diffusion pump.

Summary of the invention The present invention provides a diffusion pump characterized in that an annular heater is provided around the outer circumference of the boiler in such a manner that the level of the lower end of the heater is higher than the bottom of the boiler, the central zone of the heater is not lower than the average pump fluid level, and the length in the radial direction of the surface of pump fluid is substantially greater than the average depth of the pump fluid, thus effecting vigorous swaying and rotatory motions of the fluid.

Brief description of the drawings FIG. 1 is a longitudinal cross sectional view of one embodiment of diffusion pump of the present invention;

FIG. 2 is a graph of pressure fluctuations recorded by operation of a diffusion pump of the present invention; and

FIG. 3 is a graph of pressure fluctuations recorded by an operation of a conventional diffusion pump.

Detained description Detailed description will hereinafter be given of the present invention with reference to the drawings. In the pump shown in FIG. 1, in an outer cylinder 4 connected with boilers 2 and 2' holding pump fluids 1 and 1' in such a manner that the average depth of the fluid is shallow in comparison with the length in radial direction of the fluid surface therein and having outside cooling jacket 3, there are coaxially assembled inner cylinders 6 and 6' defining at their tops respective jet nozzles 5 and 5'. The inner cylinder 6' is fixed to the outer cylinder 4 by supporting means 7. The lower end of inner cylinder 6 is enlarged with sufficient opening between said cylinder and outer cylinder 4 to allow the pump fluid to flow down along the outside of said inner cylinder to the boiler 2'. That is, the vapor evaporated from boilers 2 and 2 and issuing from jet nozzles 5 and 5' is cooled by the cooling jacket 3 subsequently flows back to the boiler 2'. Another inner cylinder 6 is fixed to the aforesaid inner cylinder 6 by another supporting means 7, and the lower end of said inner cylinder 6 is connected with boiler 2 so as to pass the vapor evaporating from said boiler 2. An opening 8 is provided at a certain point of said connection in order to allow pump fluid 1 overflowing from boiler 2' to enter boiler 2. The upper part of inner cylinder 6 is provided with conical part 9 in such a manner that the jet nozzle 5' is formed between conical part and the open end of inner cylinder 6'. The open upper end of the other inner cylinder 6 is provided with a cap 10 fixed to said inner 6 by another supporting means 7" in such a manner that the top jet nozzle 5 is formed between them. Heaters 11 and 11' are placed in an annular form on the outer periphery of boilers 2 and 2 and these heaters are positioned in such a way that the level of the lower end of the heater is higher than the bottom of the respective boiler and that the central zone of each heater is at the same level as the liquid level in boilers 2 and 2' respectively. The upper part 12 of outer cylinder 4 is connected with the vessel to be evacuated, and a branch pipe 13 is provided on the outer cylinder 4 below the cooling jacket 3, and is connected with an auxiliary mechanical pump (not shown) in order to pump out the gas molecules. The cooling jacket 3 has an outlet (or inlet) for cooling water 14, and the outside of heaters 11 and 11' are entirely enclosed by heat-insulating layers 15.

Since the diffusion pump constructed as described above has a shallow pumping fluid in the boilers and heating is applied from the upper periphery of the fluid, there occurs when it is operated, a large discrepancy in temperature between the fluid nearer to the axis of the boiler and that at the periphery and the pump fluid in the boiler starts swaying and rotatory motions around the central axis of the boiler and is vigorously stirred, so that the phenomenon of eruptive boiling can be completely eliminated. It is found that when the side walls of the boilers are heated, evaporation rate from the liquid close to the side walls is greater than any other portion of it, and a reaction force occurs in such a manner that the liquid surface in the vicinity of the periphery of the boiler is pushed down as a whole, constituting a motive force for the swaying and rotatory motions of the fluid. Owing to the shallowness of the pumping fluid charged in the boiler, once the pump fluid starts swaying and rotatory motions, every part of the heated side walls is periodically brought into or out of contact with said fluid, accumulating heat while out of contact, and releasing the accumulated heat to the fluid while in contact, and thus the swaying and rotatory motions of the fluid may be maintained permanently, resulting in the prevention of eruptive boiling.

FIG. 2 is a diagram indicating pressure fluctuations with time when silicone oil, i.e., pentaphenyl trimethyl trisiloxane was used as pump fluid in a glass diffusion pump according to the present invention, consisting of two boilers arranged one atop another as shown in FIG. 1, in which the boiler had an inner diameter of 100 mm. at the bottomand the liquid phase had an average thickness of 15 mm., showing that a linearly constant high vacuum was continuously maintained.

In contrast, FIG. 3 is a similar diagram when the same diffusion pump was heated from the bottom in accordance with the conventional method, showing prominent peaks in pressure at repetitive intervals.

The position of each side heater is to be located in such a manner, as described in the above examples, that the level of the lower end of the heater is higher than the bottom of the respective boiler and that the central zone of the heater is not lower than the average surface level which is filled in the boiler at the depth of around 15 mm., and it has been found by experiments that the most favorable result can be obtained when the center line of the side heater is positioned somewhat higher than the average level of said fluid. It should be also noted that for causing vigorous swaying and rotatory motions, the length in the radial direction of the fluid surface is preferred to be substantially long in comparison with the average depth of the fluid, and that when the lower end of the heater is at the same level or lower than the bottom of the pump fluid, the swaying and rotatory motions cannot be sufficiently conducted.

The foregoing embodiment involves two boilers arranged one atop another. However, the present invention is also applicable to other diffusion pumps of vertical type having a single boiler or more than two boilers. The bttom of the lowest boiler may be flat like that of the con ventional unit as shown by a solid line in FIG. 1, or the bottom Wall can be spherically raised as illustrated by dotted lines in FIG. 1, reducing the amount of fluid to be initially charged into the pump. Moreover, such a raised form of bottom makes the bottom wall stronger against the vacuum within the boiler than a flat bottom, thus making it possible to reduce the wall thickness. Besides, the installation of a cooling jacket on the outside of said raised bottom becomes possible and this enables the temperature of the pump fluid to be rapidly lowered when vacuum is broken in order to stop and separate the pumping system.

As mentioned above, the diffusion pump offers extremely stable high vacuum in spite of its simple construction, and also completely prevents the pump fluid from being overheated, resulting in less requirements of heating energy than the conventional pump, and also reduces the thermal decomposition of the pump fluid. Furthermore, it enables heating operation to be controlled with far greater ease than the conventional pump in which heating operation must be controlled very carefully in order to avoid the eruptive boiling of the fluid.

It will be understood that various changes and modifications may be made without departing from the scope of the invention as defined in the appended claims. It is intended therefore that all the matter contained in the foregoing description and in the drawings are to be interpreted as illustrative only and not as limitative of the invention.

What is claimed is:

1. In a diffusion pump wherein vapor evaporated from boiling pump fluid in a boiler therein is caused to spurt out of a jet nozzle, thereby compressing gas molecules being pumped out by an auxiliary pump, and said vapor further is cooled and liquefied for return to said boiler, the improvement which comprises a boiler holding pump fluid, said boiler having a length in the radial direction of the surface of pump fluid substantially longer than the average depth of the fluid and a heater arranged in an annular form around the outside periphery of said boiler with the level of the lower end of said heater higher than the bottom of the boiler, the central zone of said heater being not lower than the average surface level of said fluid, said heater when in operation causing a rotary motion in said fluid within said boiler.

2. The diffusion pump claimed in claim 1, in which the heater is so arranged that its central zone is higher than the average surface level of the pumping fluid.

3. The diflusion pump claimed in claim 1 in which the boiler is about mm. in an inner diameter at the bottom and pump fluid charged is about 15 mm. in the average depth.

References Cited UNITED STATES PATENTS 1,775,531 9/1930 Kramer 23010l X 1,791,105 2/1931 Seitz 230-101 2,366,277 1/1945 Madine 230101 3,075,687 1/1963 Stevenson 230101 3,245,609 4/1966 Rangabe 230101 X 3,251,537 5/1966 Bancroft et al. 230-101 3,258,196 6/1966 Knox et al. 230101 3,272,965 7/1966 Reichelt 230-101 X 3,360,188 12/1967 Stuffer 230-l01 DONLEY I. STOCKING, Primary Examiner.

WARREN J. KRAUSS, Assistant Examiner. 

